Stainless steel is considered to be the most appropriate material for the fabrication of plant and equipment in the food and beverage industry. The apparent ease by which the surface finish of the material can be kept hygienically clean is a key factor in favour of stainless steel over other materials and those with applied surfaces, as is its resistance to corrosion. The manufacture of stainless steel and subsequent processes to fabricate the finished product into plant or equipment are well researched and documented.
The relevance of a so called clean surface during manufacture and fabrication is documented in international standards to ensure that the material is protected against corrosion to prevent failures during the life cyle of the product. The importance of cleanability of specific surface finishes on stainless steel and comparative materials has been researched in small scale experimental set-ups. The results indicate that the rougher surfaces make cleaning more difficult and that the cleaning processes have a significant impact on the final cleanliness of the surface. No research has been documented on the effect of the operational environment in a brewery on the passive oxide layer of the stainless steel equipment surface. The possible breakdown of the passive layer on the surface is generally known to cause corrosion, that in turn causes failures in the fabricated equipment.
The critical importance of maintaining strict hygienic standards in food processing plants has been the focus of international standard bodies to reduce the incidents of foodborne diseases. It has therefore become critical to understand how clean is the surface, and how the surface can be measured in an operational environment by using effective and reliable non-destructive testing procedures.
A brewery operational environment review of stainless steel equipment was carried out to assess the impact on the surface of the equipment after 10 years. The results obtained from this review were used to design the experimental set-up. The test vessel is a fermenter that forms part of a training brewery that produces beer using standard processes. The internal surfaces of the fermenter were prepared with 4 different finishes (2B milled, 120 and 240 mechanically polished and electropolished). These are finishes that are used in the food and beverage industry. The fermentation process carried out in the vessel created a standard soil that was then cleaned off by the standard Cleaning-In-Place (CIP) process using caustic, acid and sterilant regimes. The experiments were repeated to assess the results of the comparative cleanability on the different surfaces and the possible changes occuring on the surfaces during the fermentation and cleaning cycles. The method used to check for cleanability is based on ATP Bioluminescence that detects minute traces of organic material that indicate the level of hygiene in the vessel. The methods used to check the surface roughness include standard Two-Dimensional Profilometry directly on the metal surfaces and Three-Dimensional Microscopy on replicated samples. Visual appraisal of cleanability was also done at each step of the process. All these tests were carried out on the surfaces before use and after each fermentation and CIP cycle.
The results indicate that all surfaces are equally clean in areas where the CIP chemicals impinge directly on the surface at the top of the vessel. As the chemicals flow down the side of the vessel and reach the bottom cone, the levels of hygiene reduce. The surface that achieved the best level of hygiene is the electropolished surface finish, even at the lower section of the vessel. The mechanically polished surface (240 Grit) started to pit after the second cycle. Both the lower cleanability of surfaces at the bottom of vessel, and the roughening caused by pitting, have been observed during the operational review. The results also indicate that further work can be done to optimize the CIP processes to achieve effective cleaning at the lowest cost, and that the surface breakdown can be assessed and analysed using the replicating sample method with microscopy to determine the extent of change over the life cycle of the equiment.